People have been talking about trusting our “gut instincts” since the beginning of time and it has always meant that humans should trust that instinctual part of them that tells them if something is right or wrong. This old adage hasn’t really been backed by science, but researchers from the University of California, Los Angeles have found new meaning for the term by finding a link between the microbes in our intestines and the way our brains are formed and how they function.

Scientists at the university looked at 40 different women and performed two tests, one involving their fecal matter and the other using a magnetic resonance imagine (MRI) scanner. When inside the MRI scanner, the women were shown various images of individuals, environments, situations or objects that were designed to evoke emotional responses.

Credit: Andrew Rae

Of the 40 women tested, 33 of them had more of the bacterium called Bacterioids, so they were grouped together in the first primary group. The remaining 7 women had more of the Prevotella bacteria. The Bacterioids group showed that they had greater thickness of the gray matter found in the frontal cortex and insula, which are regions involved in complex processing of information. This group also had larger volumes of the hippocampus, which is involved in memory processing. These women were less likely to experience negative emotions when shown negative images.
By contrast, the Prevotella group had very different results. These women displayed more connections between emotional, attentional, and sensory brain regions and lower brain volumes in several regions, like the hippocampus. This group’s hippocampus was less active than the other group’s while viewing negative images and they exhibited higher levels of negative feelings related to anxiety, distress, and irritability than the Bacterioids group did.

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“Reduced hippocampal engagement to negative imagery may be associated with increased emotional arousal,” the authors concluded in their paper.

This study, though only the first piece of a very complex puzzle, supports what has been found in mice and even monkeys since the 1900s. Early scientists used to prescribe dairy products in order to change the bacterial ecology of a person experiencing depression, fatigue, and neuroses.

While some critics point out that it’s likely that a patient’s brain is what produced certain microbes in the gut, recent research has shown that this may not be true. In a study conducted on mice, the mice were fed broth infused with Lactobacillus rhamnosus, a common bacterium that is found in humans and also used to ferment milk into probiotic yogurt for several weeks. This type of bacteria is known to release immense quantities of GABA, which calms nervous activity and is targeted by high-profile anti-anxiety drugs like Xanax and Valium. After being fed this concoction, the mice were thrown into a forced-swim test in which they swam longer and experienced shorter periods of despair, showing that they were less inclined to experience depression and hopelessness. The fact that the mice reacted this way after being fed the bacteria may prove that the microbes affect brain activity, not the other way around.

While the most recent study involving the women is just the beginning of many studies to come, it did yield interesting results that are likely to further this field of study and could be the precursor to exciting things, like bacterium transplants rather than intensive psychotherapy treatments. The fact that such a link between seemingly unconnected organisms exists is reason enough to encourage further research.